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Move syncscan.c to src/backend/access/common.

Browse Source 
Since the tableam.c code needs to make use of the syncscan.c routines
itself, and since other block-oriented AMs might also want to use it one
day, it didn't make sense for it to live under src/backend/access/heap.

Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CA%2BhUKGLCnG%3DNEAByg6bk%2BCT9JZD97Y%3DAxKhh27Su9FeGWOKvDg%40mail.gmail.com
This commit is contained in:
Thomas Munro
2020-07-29 16:46:58 +12:00
parent c49c74d192
commit cb04ad4985
9 changed files with 33 additions and 11 deletions
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1
src/backend/access/heap/Makefile
View File

@@ -20,7 +20,6 @@ OBJS = \
hio.o \
pruneheap.o \
rewriteheap.o \
syncscan.o \
vacuumlazy.o \
visibilitymap.o

1
src/backend/access/heap/heapam.c
View File

@@ -41,6 +41,7 @@
#include "access/parallel.h"
#include "access/relscan.h"
#include "access/subtrans.h"
#include "access/syncscan.h"
#include "access/sysattr.h"
#include "access/tableam.h"
#include "access/transam.h"

1
src/backend/access/heap/heapam_handler.c
View File

@@ -24,6 +24,7 @@
#include "access/heaptoast.h"
#include "access/multixact.h"
#include "access/rewriteheap.h"
#include "access/syncscan.h"
#include "access/tableam.h"
#include "access/tsmapi.h"
#include "access/xact.h"

322
src/backend/access/heap/syncscan.c
View File

@@ -1,322 +0,0 @@
/*-------------------------------------------------------------------------
*
* syncscan.c
* heap scan synchronization support
*
* When multiple backends run a sequential scan on the same table, we try
* to keep them synchronized to reduce the overall I/O needed. The goal is
* to read each page into shared buffer cache only once, and let all backends
* that take part in the shared scan process the page before it falls out of
* the cache.
*
* Since the "leader" in a pack of backends doing a seqscan will have to wait
* for I/O, while the "followers" don't, there is a strong self-synchronizing
* effect once we can get the backends examining approximately the same part
* of the table at the same time. Hence all that is really needed is to get
* a new backend beginning a seqscan to begin it close to where other backends
* are reading. We can scan the table circularly, from block X up to the
* end and then from block 0 to X-1, to ensure we visit all rows while still
* participating in the common scan.
*
* To accomplish that, we keep track of the scan position of each table, and
* start new scans close to where the previous scan(s) are. We don't try to
* do any extra synchronization to keep the scans together afterwards; some
* scans might progress much more slowly than others, for example if the
* results need to be transferred to the client over a slow network, and we
* don't want such queries to slow down others.
*
* There can realistically only be a few large sequential scans on different
* tables in progress at any time. Therefore we just keep the scan positions
* in a small LRU list which we scan every time we need to look up or update a
* scan position. The whole mechanism is only applied for tables exceeding
* a threshold size (but that is not the concern of this module).
*
* INTERFACE ROUTINES
* ss_get_location - return current scan location of a relation
* ss_report_location - update current scan location
*
*
* Portions Copyright (c) 1996-2020, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
* IDENTIFICATION
* src/backend/access/heap/syncscan.c
*
*-------------------------------------------------------------------------
*/
#include "postgres.h"
#include "access/heapam.h"
#include "miscadmin.h"
#include "storage/lwlock.h"
#include "storage/shmem.h"
#include "utils/rel.h"
/* GUC variables */
#ifdef TRACE_SYNCSCAN
bool trace_syncscan = false;
#endif
/*
* Size of the LRU list.
*
* Note: the code assumes that SYNC_SCAN_NELEM > 1.
*
* XXX: What's a good value? It should be large enough to hold the
* maximum number of large tables scanned simultaneously. But a larger value
* means more traversing of the LRU list when starting a new scan.
*/
#define SYNC_SCAN_NELEM 20
/*
* Interval between reports of the location of the current scan, in pages.
*
* Note: This should be smaller than the ring size (see buffer/freelist.c)
* we use for bulk reads. Otherwise a scan joining other scans might start
* from a page that's no longer in the buffer cache. This is a bit fuzzy;
* there's no guarantee that the new scan will read the page before it leaves
* the buffer cache anyway, and on the other hand the page is most likely
* still in the OS cache.
*/
#define SYNC_SCAN_REPORT_INTERVAL (128 * 1024 / BLCKSZ)
/*
* The scan locations structure is essentially a doubly-linked LRU with head
* and tail pointer, but designed to hold a fixed maximum number of elements in
* fixed-size shared memory.
*/
typedef struct ss_scan_location_t
{
RelFileNode relfilenode; /* identity of a relation */
BlockNumber location; /* last-reported location in the relation */
} ss_scan_location_t;
typedef struct ss_lru_item_t
{
struct ss_lru_item_t *prev;
struct ss_lru_item_t *next;
ss_scan_location_t location;
} ss_lru_item_t;
typedef struct ss_scan_locations_t
{
ss_lru_item_t *head;
ss_lru_item_t *tail;
ss_lru_item_t items[FLEXIBLE_ARRAY_MEMBER]; /* SYNC_SCAN_NELEM items */
} ss_scan_locations_t;
#define SizeOfScanLocations(N) \
(offsetof(ss_scan_locations_t, items) + (N) * sizeof(ss_lru_item_t))
/* Pointer to struct in shared memory */
static ss_scan_locations_t *scan_locations;
/* prototypes for internal functions */
static BlockNumber ss_search(RelFileNode relfilenode,
BlockNumber location, bool set);
/*
* SyncScanShmemSize --- report amount of shared memory space needed
*/
Size
SyncScanShmemSize(void)
{
return SizeOfScanLocations(SYNC_SCAN_NELEM);
}
/*
* SyncScanShmemInit --- initialize this module's shared memory
*/
void
SyncScanShmemInit(void)
{
int i;
bool found;
scan_locations = (ss_scan_locations_t *)
ShmemInitStruct("Sync Scan Locations List",
SizeOfScanLocations(SYNC_SCAN_NELEM),
&found);
if (!IsUnderPostmaster)
{
/* Initialize shared memory area */
Assert(!found);
scan_locations->head = &scan_locations->items[0];
scan_locations->tail = &scan_locations->items[SYNC_SCAN_NELEM - 1];
for (i = 0; i < SYNC_SCAN_NELEM; i++)
{
ss_lru_item_t *item = &scan_locations->items[i];
/*
* Initialize all slots with invalid values. As scans are started,
* these invalid entries will fall off the LRU list and get
* replaced with real entries.
*/
item->location.relfilenode.spcNode = InvalidOid;
item->location.relfilenode.dbNode = InvalidOid;
item->location.relfilenode.relNode = InvalidOid;
item->location.location = InvalidBlockNumber;
item->prev = (i > 0) ?
(&scan_locations->items[i - 1]) : NULL;
item->next = (i < SYNC_SCAN_NELEM - 1) ?
(&scan_locations->items[i + 1]) : NULL;
}
}
else
Assert(found);
}
/*
* ss_search --- search the scan_locations structure for an entry with the
* given relfilenode.
*
* If "set" is true, the location is updated to the given location. If no
* entry for the given relfilenode is found, it will be created at the head
* of the list with the given location, even if "set" is false.
*
* In any case, the location after possible update is returned.
*
* Caller is responsible for having acquired suitable lock on the shared
* data structure.
*/
static BlockNumber
ss_search(RelFileNode relfilenode, BlockNumber location, bool set)
{
ss_lru_item_t *item;
item = scan_locations->head;
for (;;)
{
bool match;
match = RelFileNodeEquals(item->location.relfilenode, relfilenode);
if (match || item->next == NULL)
{
/*
* If we reached the end of list and no match was found, take over
* the last entry
*/
if (!match)
{
item->location.relfilenode = relfilenode;
item->location.location = location;
}
else if (set)
item->location.location = location;
/* Move the entry to the front of the LRU list */
if (item != scan_locations->head)
{
/* unlink */
if (item == scan_locations->tail)
scan_locations->tail = item->prev;
item->prev->next = item->next;
if (item->next)
item->next->prev = item->prev;
/* link */
item->prev = NULL;
item->next = scan_locations->head;
scan_locations->head->prev = item;
scan_locations->head = item;
}
return item->location.location;
}
item = item->next;
}
/* not reached */
}
/*
* ss_get_location --- get the optimal starting location for scan
*
* Returns the last-reported location of a sequential scan on the
* relation, or 0 if no valid location is found.
*
* We expect the caller has just done RelationGetNumberOfBlocks(), and
* so that number is passed in rather than computing it again. The result
* is guaranteed less than relnblocks (assuming that's > 0).
*/
BlockNumber
ss_get_location(Relation rel, BlockNumber relnblocks)
{
BlockNumber startloc;
LWLockAcquire(SyncScanLock, LW_EXCLUSIVE);
startloc = ss_search(rel->rd_node, 0, false);
LWLockRelease(SyncScanLock);
/*
* If the location is not a valid block number for this scan, start at 0.
*
* This can happen if for instance a VACUUM truncated the table since the
* location was saved.
*/
if (startloc >= relnblocks)
startloc = 0;
#ifdef TRACE_SYNCSCAN
if (trace_syncscan)
elog(LOG,
"SYNC_SCAN: start \"%s\" (size %u) at %u",
RelationGetRelationName(rel), relnblocks, startloc);
#endif
return startloc;
}
/*
* ss_report_location --- update the current scan location
*
* Writes an entry into the shared Sync Scan state of the form
* (relfilenode, blocknumber), overwriting any existing entry for the
* same relfilenode.
*/
void
ss_report_location(Relation rel, BlockNumber location)
{
#ifdef TRACE_SYNCSCAN
if (trace_syncscan)
{
if ((location % 1024) == 0)
elog(LOG,
"SYNC_SCAN: scanning \"%s\" at %u",
RelationGetRelationName(rel), location);
}
#endif
/*
* To reduce lock contention, only report scan progress every N pages. For
* the same reason, don't block if the lock isn't immediately available.
* Missing a few updates isn't critical, it just means that a new scan
* that wants to join the pack will start a little bit behind the head of
* the scan. Hopefully the pages are still in OS cache and the scan
* catches up quickly.
*/
if ((location % SYNC_SCAN_REPORT_INTERVAL) == 0)
{
if (LWLockConditionalAcquire(SyncScanLock, LW_EXCLUSIVE))
{
(void) ss_search(rel->rd_node, location, true);
LWLockRelease(SyncScanLock);
}
#ifdef TRACE_SYNCSCAN
else if (trace_syncscan)
elog(LOG,
"SYNC_SCAN: missed update for \"%s\" at %u",
RelationGetRelationName(rel), location);
#endif
}
}